1. Field of the Invention
The present invention relates to an apparatus for manufacturing cathode-ray tubes, and more particularly to a cathode-ray tube manufacturing apparatus capable of automatically evacuating and sealing a cathode-ray tube bulb after electron guns have been sealingly installed in the neck of the cathode-ray tube bulb.
2. Description of the Related Art
In the manufacture of cathode-ray tubes such as color cathode-ray tubes, for example, as shown in FIG. 1 of the accompanying drawings, electron guns 3 are inserted in a neck 2 of a cathode-ray tube bulb 1, and a glass stem 6 integral with an evacuating pipe (tipoff pipe) 4 and from which step pins 5 are extended is sealingly mounted in the end of the neck 2. Thereafter, the cathode-ray tube bulb 1 is delivered to an evacuating and sealing process in which it is evacuated and the evacuating pipe 4 is melted and sealed.
In the evacuating and sealing process, the cathode-ray tube bulb 1 which has been fed from the electron gun sealing process by a bulb conveyor is transferred onto an evacuating device, which will hereinafter be referred to as an evacuating cart, on evacuating furnace rails by an automatic transfer mechanism.
The evacuating cart is equipped with an evacuating port for connecting the evacuating pipe 4 to an evacuating pump and a sealing jig for melting with heat and sealing the evacuating pipe 4 after the cathode-ray tube bulb 1 has been evacuated.
The cathode-ray tube bulb 1 is set on the evacuating cart such that the evacuating pipe 4 is inserted through the sealing jig into the evacuating port. The cathode-ray tube bulb 1 is carried on the evacuating cart along the evacuating furnace rails into an evacuating furnace. In the evacuating furnace, the cathode-ray tube bulb 1 is heated to a predetermined temperature and evacuated, and thereafter the evacuating pipe 4 is sealed.
When the above process is finished, the evacuating cart is delivered out of the evacuating furnace, and the evacuating port and the sealing jig are removed from the evacuating cart. The cathode-ray tube bulb 1 is dismounted from the evacuating cart by the automatic transfer mechanism, and then conveyed to a next process. The evacuating cart is circulated into the evacuating furnace with another cathode-ray tube bulb mounted thereon.
In the art of manufacturing cathode-ray tubes, it has been desired to fully automatically evacuating and sealing cathode-ray tube bulbs.
However, there are actually instances wherein cathode-ray tube bulbs cannot be charged onto all evacuating carts in the sequence described above. Since it is necessary to subject all the evacuating carts to vacuum aging in the evacuating furnace, i.e., to keep the evacuating pump of each evacuating cart always in an operative condition even if no cathode-ray tube bulb is charged onto the evacuating cart, the worker inserts a dummy evacuating pipe into the evacuating port of any evacuating cart that is charged with no cathode-ray tube bulb. Since the evacuating carts are stopped in somewhat different positions, it is difficult to automatically insert and remove dummy pipes, and hence the worker is relied upon for inserting and removing dummy evacuating pipes.
Fully automatic execution of the evacuating and sealing process requires that dummy evacuating pipes be automatically inserted into and removed from corresponding evacuating ports.
After a cathode-ray tube bulb has been sealed, the evacuating port and the sealing jig are removed by the worker. If the evacuating pipe is broken and left in the evacuating port, it is the current practice for the worker to remove the broken evacuating pipe. Consequently, in order that the evacuating and sealing process can be fully automated, it is necessary to automatically detect whether the evacuating pipe is broken or not after the cathode-ray tube bulb has been evacuated and sealed.
In the evacuating and sealing process, the state of an evacuating cart is varied several times in the evacuating furnace by limit switches that are mounted on the evacuating cart. Heretofore, the limit switches on the evacuating cart have been operated by rod-shaped switch changers. In the event that a switch changer interferes with a rotating or fixed portion of the limit switch, it has been customary for the worker to take care of such an accident.
Fully automatic execution of the evacuating and sealing process also requires that such an accident be detected early.
Even if cathode-ray tube bulbs charged onto respective evacuating carts are processed in the predetermined processing sequence, not all the cathode-ray tube bulbs can necessarily be discharged as good or acceptable products from the evacuating and sealing process. One of the reasons for the problem is a failure of the sealing jig.
One conventional device for detecting a sealing jig failure is incorporated in a sealing device, and capable of detecting both a wire break and a full short circuit. However, it has not been customary to detect a short circuit across one turn or the like of the coil heater of the sealing jig because any change in the coil resistance due to such a short circuit is very small. Even when the coil heater of a sealing jig suffers such a short circuit, therefore, it has been the practice to keep the cathode-ray tube bulb processed. With such a coil heater short circuit, the evacuating pipe is sealed at a voltage different from a predetermined voltage pattern owing to a resistance change caused by the short circuit, resulting in a poor sealing condition of the evacuating pipe. It has thus been necessary for the worker to visually check sealed evacuating pipes for a sealing failure.
If any coil heater short circuit occurs across only one turn or the like, the sealed condition of the evacuating pipe differs only slightly from the normally sealed condition. Consequently, since sealed cathode-ray tube bulbs are checked by the worker, it has been difficult to uniquely determine whether the cathode-ray tube bulbs have been sealed properly or not.
When a possibility of failure of any of the sealing jigs is recognized, it has been customary for the worker to measure the resistance of the sealing jig with a resistance measuring instrument or the like to determine whether it actually suffers a failure or not.
Other requirements for complete automatization of the evacuating and sealing process are early automatic detection of a sealing failure of evacuating pipes and a reduction in the burden on the worker in confirming the sealed condition of evacuating pipes.